What is a measure of obesity?

Measures of Obesity

Body Mass Index (BMI) is the primary measure used to define and screen for obesity, calculated as weight in kilograms divided by height in meters squared (kg/m²), with obesity defined as a BMI ≥30 kg/m². 1, 2

BMI Calculation and Classification

BMI serves as the standard screening tool for obesity assessment in clinical practice and epidemiological studies. 1

Calculation Methods:

• Metric formula: weight (kg) / height (m)² 1
• Imperial formula: [weight (pounds) / height (inches)²] × 703 1

Standard Classification (Adults):

• Underweight: BMI <18.5 kg/m² 2
• Normal weight: BMI 18.5-24.9 kg/m² 2
• Overweight: BMI 25.0-29.9 kg/m² 1, 2
• Obesity: BMI ≥30.0 kg/m² 1, 2
  • Class 1 (mild): BMI 30-34.9 kg/m² 2
  • Class 2 (moderate): BMI 35-39.9 kg/m² 2
  • Class 3 (severe): BMI ≥40 kg/m² 2

Ethnic-Specific Considerations:

Asian populations require lower BMI thresholds due to greater adiposity and higher comorbidity risks at lower BMI levels, with overweight defined as BMI ≥23 kg/m² in this population. 1, 2

Strengths of BMI

BMI demonstrates excellent reliability and correlation with body fat:

• Highly correlated with body fat percentage: R² = 0.95 in men; R² = 0.98 in women 1
• Easy to measure and reproducible in clinical settings 1
• Strong prospective links with adverse health outcomes including cardiovascular disease, diabetes, and mortality 1
• Good specificity (90%) for detecting excess adiposity 1

Critical Limitations of BMI

BMI has poor sensitivity (50%) for identifying excess adiposity, missing half of individuals with excess body fat. 1

Key Limitations:

• Does not distinguish between lean and fat mass, potentially misclassifying muscular individuals as overweight 1, 2
• Does not account for body fat distribution, an independent cardiovascular risk factor 1
• Age and sex variations: Women have higher body fat percentages than men at similar BMI levels 1
• Ethnic differences: Hispanic women have higher body fat than Black and White women at similar BMI; Black women have lower body fat than White women at the same BMI 1
• 30-46% of individuals with BMI <30 kg/m² have obesity-level body fat when measured directly 1, 3

Complementary Measures for Central Adiposity

Waist circumference should be measured in addition to BMI to assess central (abdominal) adiposity, which predicts cardiovascular risk independent of BMI. 1, 2

Waist Circumference Thresholds:

• Men: >102 cm (>40 inches) indicates increased cardiovascular risk 2
• Women: >88 cm (>35 inches) indicates increased cardiovascular risk 2

Additional Anthropometric Measures:

• Waist-to-hip ratio: Better predictor of cardiovascular and total mortality than BMI in some populations 1, 2
• Waist-to-height ratio: Useful for identifying metabolic abnormalities 2

Central adiposity measured by waist circumference predicted cardiovascular mortality in older men (≥65 years) when BMI showed no relationship with mortality. 1

Pediatric Considerations

Children require age- and sex-specific BMI percentiles rather than absolute BMI cutoffs used in adults. 4

Pediatric Classification:

• Overweight: BMI 85th-94th percentile for age and sex 4
• Obesity: BMI ≥95th percentile for age and sex 4
• Severe obesity: BMI ≥120% of the 95th percentile 4

Rapid changes in body composition during growth make obesity assessment more complex in children than adults, requiring age-adjusted reference values. 1, 4

Clinical Application Algorithm

For Adults:

1. Calculate BMI using weight and height 1
2. If BMI ≥30 kg/m²: Obesity is confirmed; proceed to risk assessment 1, 2
3. If BMI 25-29.9 kg/m²: Measure waist circumference to assess central adiposity 1, 2
4. If BMI <25 kg/m² but clinical suspicion exists: Consider direct body fat measurement, as BMI misses significant numbers of individuals with excess adiposity 1, 3
5. For Asian patients: Apply lower thresholds (BMI ≥23 kg/m² for overweight) 1, 2

Common Pitfalls:

• Relying solely on BMI without measuring waist circumference misses individuals with high-risk central adiposity 1
• Using BMI alone in elderly patients, where it correlates less strongly with body fat percentage 1
• Applying adult BMI cutoffs to children, who require percentile-based assessment 4
• Ignoring ethnic-specific differences in body composition and disease risk 1, 2